1. Field of the Invention
The invention relates to a method of manufacturing an electrode collector used as a constituent element of a battery, and an apparatus for manufacturing that electrode collector. The invention also relates to a battery provided with that collector.
2. Description of the Related Art
The demand for lithium secondary batteries (such as a lithium-ion batteries), which charge and discharge by lithium ions moving back and forth between an anode and a cathode, as on-board power supplies for vehicles and power supplies for personal computers and mobile terminals is expected to increase due to their light weight and high output. In one typical structure of this kind of secondary battery, an electrode is provided that has a structure in which material (electrode active material) capable of irreversibly storing and releasing lithium ions is retained on a conductive member (an electrode collector). For example, a typical example of electrode active material used for the anode (i.e., anode active material) is an oxide that includes lithium and one or two or more types of transition metal elements as a component metal element (hereinafter this oxide may also be referred to as a “lithium transmission metal oxide”). Also, another typical example of an electrode collector used for the anode (i.e., anode collector) is a sheet or foil-like member of mainly aluminum or an aluminum alloy.
An anode collector made of aluminum or an aluminum alloy oxidizes easily. For example, when exposed to air, the surface of an anode collector made of aluminum or an aluminum alloy immediately oxidizes and therefore always has an oxide film. When the surface of the collector is covered by an oxide film, the oxide film acts as an insulating film (such as an insulating film having a resistivity of 1014Ω×cm) so the interface resistance between the anode collector and the anode active material increases. Japanese Patent Application Publication No. 11-250900 (JP-A-11-250900), for example, describes technology for preventing the formation of such an oxide film on the surface of a collector. The technology described in JP-A-11-250900 forms an overcoat layer from highly conductive material such as carbon (e.g., a carbon overcoat layer) on a collector using an electron beam evaporation apparatus, while removing the oxide film from the surface of the collector by etching the collector (aluminum foil) surface using a sputter ion beam etching apparatus.
However, with the technology described in JP-A-11-250900, when etching the collector (aluminum foil) surface, sputter ion beam etching is performed using an ion beam of an inert gas, which reduces productivity of the electrode collector. That is, an ion beam of an inert gas has a low etching rate so etching must be performed for a long time to reliably remove the oxide film from the surface of the collector, which is undesirable from the viewpoint of productivity.
For example, according to the results of tests performed by the inventors of this application, when a plasma treatment is performed while supplying argon (Ar) gas at a flowrate of 15 ml/min with the pressure of the atmosphere being 0.1 Pa, and 200 W of sputtering power using a common sputtering apparatus when the oxide film on the aluminum foil surface is approximately 5 nm thick, it took five minutes to remove the oxide film from the aluminum foil surface and the etching rate at that time was around 1 nm/min. Upon examination by the inventors of this application, it was deduced that an etching rate of at least 20 nm/min to 40 nm/min is necessary to continuously perform the etching process in-line suitable for continuous manufacture. Therefore, with the electrode collector manufacturing apparatus according to the related art described in JP-A-11-250900, it is difficult to perform the foregoing etching process and carbon process successively in-line.